The present invention relates to an automatic mechanical guide system for one or more torches of a welding unit, for example an arc-welding unit, in a groove delimited between the two bevelled end surfaces of two metal parts to be welded together, of the type comprising a main bug that moves along a fixed guide substantially parallel to the groove, a secondary bug hinged and attached to the main bug and equipped with a torch holder and a guide device comprising roller components in contact with the surfaces of the metal parts bordering the groove and tracer components that enter the groove, said device comprising two tracer components that are respectively in contact with the two side walls of the groove and are respectively located towards the front and rear of the secondary bug in the direction of movement of the main bug.
Generally, an automatic welding machine involves a large number of operating parameters linked to the welding process used, the geometric characteristics of the groove delimited between the two parts to be welded and the welding conditions (nature, shape and position of the parts, etc.). In most industrial applications, the welding operation takes place in several passes, and obtaining a high-quality weld depends on the accuracy with which the first pass is made. It is therefore important that the end surfaces that delimit the groove between them are machined as evenly as possible and that a system capable of following the geometry of the groove is obtained so that the weld is applied very accurately in the bottom of and along the axis of the groove.
It is common practice to machine a bevel at the two ends of the parts to be welded. Depending on the angle of the bevel, the groove obtained is flared to a greater or lesser extent; a wide groove increases the number of passes and the welding time and a narrow groove, while it does not present these drawbacks, also poses problems of accuracy for the guide system.
Thus, even with such machining, the groove delimited by the two bevels cannot have constant geometric characteristics along its entire length. Once the two end surfaces have been placed opposite each other, the manufacturing tolerances of the parts and the machining tolerances of the bevels will necessarily lead to variations in width and depth in the groove. If the two parts to be welded are relatively long tubes, for example, with a bevel machining tolerance of +/−0.1 mm, this results in a groove width to within +/−0.2 mm with depth of 2 to 3 mm due in particular to the ovalisation of the tubes.
If the welding process is gas-shielded electric arc welding, the welding electrodes held by the torches must enter the groove and be located a given distance from the walls and the bottom of the groove or the last weld layer deposited. Under these conditions, the accuracy of the guide system is particularly important, especially when this welding process is used in a narrow groove.
Consequently, the accuracy of the weld, particularly on the first pass, involves accurate positioning of the welding electrodes, not only relative to the mid-plane of the groove, but also relative to the bottom of the groove. The more accurate the guide system, the more these two positions, respectively sensitive to variations in groove width and depth, will be observed.
To this end, EP 0439975 proposes a guide system that can accurately ensure the positioning of the welding torches inside the groove and is capable of taking into account the width and depth variation of the groove and thus improve the quality of the weld required in certain applications, particularly for the end-to-end welding of sections of tube forming pipelines.
Thus, the automatic mechanical guide system for one or more torches on an arc welding unit in a groove delimited by the bevelled end surfaces of two metal parts to be welded together, of the type comprising a main bug that moves along a fixed guide parallel to the groove, a secondary bug that is connected to the main bug by a jointed connecting assembly and is equipped with a torch holder and a guide device comprising roller components in contact with the surfaces of the metal parts bordering said groove and tracer components that enter into the groove. The guide device comprises a first tracer and roller assembly that is located at the front of the secondary bug in the direction of movement of the main bug and comprises a front tracer component in contact with one of the two side walls of the groove and a front roller component rolling on the surface of one of the two metal parts on one side of the groove. The device also comprises a second tracer and roller assembly that is located at the rear of the secondary bug and comprises a rear tracer component in contact with the other side wall of the groove and a rear roller component rolling on the surface of the other metal part on the other side of the groove. The torch(es) are then mounted on the secondary bug so that they are between the first and second tracer and roller assemblies. The jointed connecting assembly comprises a first pivot axis that is perpendicular to the direction of movement of the main bug and is located in a plane parallel to the mid-plane of the groove and a second pivot axis that is perpendicular to the direction of movement of the main bug and perpendicular to the mid-plane of the groove, the first and second pivot axes being located between the first and second tracer and roller assemblies. Lateral pushing means are also provided that act on the secondary bug to urge it to pivot about the first axis so that each tracer component remains in contact with the associated wall of the groove and pushing means that constantly urge the roller components on the second bug against the metal parts respectively associated with them.
Through these different positions, the torches being arranged between the tracer components, only a small proportion of the width and depth variations of the groove are passed on to the welding electrodes so that their deviation relative to the mid-plane and the bottom of the groove is reduced to a minimum, which implies improved following of the geometric characteristics of the groove and improved accuracy of the weld.
However, it has been noted during operation that during the positioning of the secondary bug on the join plane, the machine axis is sometimes not perpendicular to the axis of the join plane, or the machine axis is sometimes not concentric with the axis of the tubes to be welded. The secondary bug is then moved along the join plane without the tracer and roller assemblies being effective, or it is necessary to reposition the entire welding device.